The are an important part of the kidney's urine-concentrating mechanism found surrounding the nephron loops (loops of henle) of juxtamedullary nephrons.

Vasa recta

Explanation:

Concentration of urine occurs by counter current mechanism. It is a mechanism to retain water in the body and it occurs by vasa recta and loop of Henle. Vasa rectae are the peritubular capillaries found near the loop of Henle (LOH). The blood flow in opposite directions in the two limbs of vasa rectae and also in the two limbs of LOH. The flow of fluid in the descending limb of LOH is in opposite direction to the flow of blood in ascending limb of vasa retae and vice versa. These counter current systems significantly contribute to the concentration of urine in the kidneys.

It is the cyclic movement of Na+, Cl- from the ascending limb of loop of Henle to the descending limb of vasa rectae via the medullary interstitial fluid. This is responsible for 5 % absorption of water in the descending limb of LOH, making the filtrate hypertonic. Due to this the concentration of Na+ and Cl- is more in the renal medulla. The osmolarity increases from 300 to 1200 mOsml/l from cortex to inner medulla, concentrating the urine.